Preliminary work on the fracture history of the southern Piceance Basin, near its western rim in the valleys of Plateau Creek and the Colorado River, suggests that five episodes of jointing occurred in Upper Cretaceous through Eocene strata. Both the relative age and style of each joint set are similar to those documented for correlative sets in Paleocene to Eocene strata farther north in the drainage areas of Piceance and Yellow Creeks (Verbeek and Grout, 1983a). Joint sets produced during each of the five episodes of fracturing are of regional extent and are collectively referred to as the "Piceance system" by Verbeek and Grout (1984a).The distribution of joints of the Piceance system relative to stratigraphic level varies laterally across the basin. The base of the Piceance system in most areas has not yet been defined and in many places can be documented only by drilling. Along the eastern margin of the basin these joints appear restricted to basin rocks of Paleocene to late Eocene age. Older, pre-basin strata of Late Cretaceous age exposed within the adjacent Grand Hogback monocline are cut only by older joints in two sets collectively termed the "Hogback system". In contrast, joints of the Hogback system are missing along the basin's western margin, where both pre-basin and basin strata are cut by the various younger sets of the Piceance system. The two joint systems thus cut across stratigraphic boundaries, complicating attempts to predict fracture patterns at depth, beneath the basin interior, from surface studies of joints prior to drilling or mining. Successful prediction necessitates both (1) that the depth of the transition zone between the Piceance and Hogback systems be known or can be reasonably estimated, and (2) that the local fracture network be well understood within the context of the regional fracture pattern.

Fractures in exposed strata of the Mesaverde Group (Late Cretaceous) and overlying Wasatch Formation (Paleocene-early Eocene) were measured and described at 137 sites within five separate areas in and around the Piceance basin. These data supplement information previously gathered at more than 300 sites in younger strata of the basin interior. The data were used to test the feasibility of predicting fracture orientations and other characteristics (dimensions, spacing, aperture) in correlative rocks at depth, as an aid to development of methods to stimulate production of natural gas from low-permeability sandstone reservoirs of the Mesaverde Group. Fractures in exposed strata of the Mesaverde Group along the Grand Hogback monocline are similar in orientation to those documented in core from the nearby MWX site, where equivalent strata are buried at depths of 1200-2500 m. The monoclinal strata, then, may be suitable for study as an exposed analog of the gas-bearing reservoir at depth. However, Mesaverde strata in DeBeque Canyon, also relatively close to the MWX site, are cut by fractures of wholly different orientation than those documented in core, and fractures in younger strata immediately surrounding the MWX site likewise do not match those found at depth. We explain these results as the effect of sampling fractures from two different fracture systems that overlap only partially in space and perhaps not at all in time. The older of the two systems is the structurally lower one, and dominates the buried Mesaverde strata at the MWX site. Equivalent fractures along the Grand Hogback monocline are exposed at the surface only as a fortuitous result of monoclinal folding. West of the monocline, at and near the MWX site, surface exposures are dominated by a structurally shallower and areally extensive system of joints previously documented in the Green River and Uinta Formations (both of Eocene age) of the northern Piceance basin. The transition zone between the two fracture systems is well exposed along the White River west of the monocline.

Zl Introduction Zl General geology Z2 Lithology Z5 Structural setting Z6 Economic significance of fractures Z6 Previous studies of joints in the Piceance basin Z7 Procedure and techniques Z7 Joint sets of the Piceance basin and Grand Hogback monocline Z8 Piceance system Z8 Hogback system Z10 Joint sets of the Divide Creek and Wolf Creek anticlines Z10 Discussion Z14 Distribution of MV2 set Z15 Presence and prominence of local fold-parallel fractures Z16 Distribution of fractures of the Piceance system Z18 Fracture history and fluid movement Z20 Timing of fracture formation Z20 Fluid movement through upper Mesaverde and lower Wasatch strata Z22 Comments on coal Z22 Concluding remarks Z22 References cited Z23 Appendix Summary of characteristics of fracture sets Z28 FIGURES 1-2. Maps showing: 1. Location of study area, late Paleozoic uplifts, and Tertiary basins and uplifts, northeastern part of Colorado Plateau and adjacent Rocky Mountain foreland Z3 2. Geology of area of Divide Creek and Wolf Creek anticlines and part of Grand Hogback monocline Z4 3. Vertical section through study area showing combined interpretation of seismic, gravity, and drill-hole data Z5 4 6. Maps showing: 4. Regional distribution and orientation of joints of Fj F4 sets in Piceance basin Z9 5. Average orientation of joints of MV2 fracture set in study area, southern Piceance basin Z12 6. Average orientation of joints of fold-parallel fracture sets in study area, southern Piceance basin Z13 7. Photographs showing fold-parallel fractures in sandstone bed of Wasatch Formation on Divide Creek anticline Z14 8. Map showing average orientation of joints of F: fracture set in study area, southern Piceance basin Z15 Contents III IV Contents

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